1. Field of the Invention
The present invention relates to apparatuses and methods for controlling resistivity of ultra pure water to be used as cleaning water in the production of semiconductor or liquid crystal devices.
This application is based on patent application No. Hei 9-130966 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
It has been widely known that when a photomask substrate is cleaned using ultra pure water (resistivity .gtoreq.18 M.OMEGA..multidot.cm) in the process of producing semiconductor or liquid crystal devices, due to the high resistivity of ultra pure water, static electricity is generated, which causes dielectric breakdown or adsorption of particulates, exerting marked undesirable effects on the production rate of the substrate.
Accordingly, in order to avert such undesirable effects a method has been generally known in which the resistivity of ultra pure water is reduced by passing the ultra pure water through magnesium meshes.
In addition, methods or apparatuses in which a module of hydrophobic porous hollow-fiber membranes is used has been proposed such as "Apparatus for Dissolving Carbon Dioxide Gas into Drinking Water" (Japanese Utility Model Application, First Publication (Kokai), No. 57-86623), "Apparatus for Controlling Resistivity of Ultra Pure Water" (Japanese Patent Application, Second Publication (Kokoku), No. Hei 5-21841), and "Method and Apparatus for Controlling Resistivity of Ultra Pure Water" (Japanese Patent Application, First Publication (Kokai), Hei 7-60082).
However, since hydrophobic porous membranes such as those made of polypropylene are used in a module according to the methods of Japanese Utility Model Application, First Publication (Kokai), No. 57-86623 and Japanese Patent Application, Second Publication (Kokoku), No. Hei 5-21841, carbon dioxide gas which has penetrated through the membranes exists as bubbles in water, not only making it difficult to control the resistivity at a constant and uniform value, but also causing problems in that water vapor condenses on the surface of the pore portions of the porous layer, and ultra pure water leaks.
Furthermore, since the flow rate of at least one of the ultra pure water to be treated and the carbon dioxide gas is controlled in the method of Japanese Patent Application, Second Publication (Kokoku), No. Hei 5-21841, the method requires an apparatus having a complicated controlling mechanism, and when the required flow rate of ultra pure water varies, and the resistivity is controlled to a predetermined value, there have been problems in that the apparatus cannot quickly respond to the variations in the required flow rate while maintaining the constant resistivity value.
Moreover, in order to solve the above problems, a three-layer membrane structure is employed in which both surfaces of a homogeneous membrane are overlaid with porous layers, and dissolved oxygen is degassed before carbon dioxide gas is supplied according to Japanese Patent Application, First Publication (Kokai), Hei 7-60082, which incurs other problems in that the apparatus and method for controlling the resistivity are complicated.